Cutting planes and representative histological sections of eye and thyroid tissues in juvenile zebrafish (Danio rerio). In the region of the gills, isolated thyroid follicles are dispersed close to the ventral aorta. The follicular shape ranges from round to oval, with cuboidal epithelium and uniformly stained colloids. Eye sections are taken at the level of the optical nerve. Sections of 2.5 μm thickness and stained with hematoxylin and eosin G (H&E)

Cluster of thyroid follicles surrounded by blood vessels in 71.3 mg/L 6-propyl-2-thiouracil (PTU)-treated juvenile F1 zebrafish (Danio rerio: B and D) and nonexposed juvenile zebrafish (A and C). Illustration of one exemplary thyroid follicle diameter measurement (C and D: blue marks) and illustration of thyroid follicle epithelium thickness measurements (C and D: yellow marks) at five locations in one follicle

Histological section of a control larval F1 zebrafish (Danio rerio) retina (A and B). Exemplary measurements of the thickness of the retinal pigment epithelium (RPE) with visible optical nerve (ON). Five out of eight measurements at different locations equally distributed across the retina are indicated (B: yellow marks)

Thyroid follicle diameter (left) and height of thyroid epithelial cells (right) in adult F0 zebrafish (Danio rerio) exposed to 6-propyl-2-thiouracil (PTU) or 17α-ethinylestradiol (EE2) for 21 days. Data are given as means ± SD (n = 19–24 adult individuals per treatment, four replicates of 5–12 individuals each). Difference from controls (analysis of variance [ANOVA], Tukey's multiple comparison test): *p < 0.05, **p < 0.01, ***p < 0.001, ****p < 0.0001

Increase in thyroid follicle diameter and epithelium height in juvenile F1 zebrafish (Danio rerio) exposed 6-propyl-2-thiouracil (PTU) or 17α-ethinylestradiol (EE2) for 60 days. Data are given as Min-to-Max plot for n = 18–65 individuals per treatment, four replicates of 15–30 individuals each. Difference from controls (analysis of variance [ANOVA], Tukey's multiple comparison test): *p < 0.05, **p < 0.01, ***p < 0.001, ****p < 0.0001

Thickness of the retinal pigment epithelium in 6-propyl-2-thiouracil (PTU)-exposed 21-day larval F1 zebrafish (Danio rerio; A), PTU-exposed 60-day juvenile F1 zebrafish (B), and 17α-ethinylestradiol (EE2)-exposed juvenile 60-day F1 zebrafish (C). No data for F1 larvae exposed to 3.9 mg/L PTU and larvae exposed to 4.9 ng/L EE2 are shown due to limitations in egg numbers. Data are given as Min-to-Max plot for n = 10 larvae or 15–60 juveniles per exposure group, four replicates of 15–30 individuals each. Difference from controls (analysis of variance [ANOVA]; Tukey's comparison test): *p < 0.05, ***p < 0.001